On the first 3 exams of his geometry class, Luis got an average score of 81. What does he need on the next exam to have an overall average of 83?
Let his score on the next exam be $x$ The sum of all of his scores is then $3 \cdot 81 + x$ The same sum must also be equal to $4 \cdot 83$ Solve: $x = 4 \cdot 83 - 3 \cdot 81 = 89$.